EP 1 571 061 A1 describes a brake control device and brake system of the general type under consideration. Such systems provide a service brake, which can be actuated by means of a brake pedal, and a parking brake (often also referred to as a handbrake), which can be actuated by means of an electric signal transducer.
The failure of the electric power supply can be a problematic event in such electrically controlled brake systems, as electric components, such as electric control systems and electrically actuated solenoid valves, can no longer be actuated. Furthermore, electric signal transducers for the parking brake also can fail as a result of such power failures. DE 199 53 805 C1 therefore proposed that emergency braking be initiated automatically by venting a spring actuator that acts on the parking brake. Automatic emergency braking can be disadvantageous, however, since it involves maximum braking action, which may present a hazard by reason of traffic following the braking vehicle.
EP 1 571 061 A1 proposed a brake system by which, in the event of failure of the electric power supply, the vehicle can be braked gradually by actuating the brake pedal under pneumatic control of the spring store parts of the spring brake cylinders. However, this solution has the disadvantage that the spring brake cylinders are repressurized as soon as the brake pedal is no longer being actuated, with the result that the parking brake is released once again. Thus, with this known system, the vehicle cannot be safely parked.